Patent GB-1069562-A (GB)
Title: Random access communications system
IPC: H04J13/00
Filed: 1964-08-04
Abstract: 1,069,562. Matched filters. INTERNATIONAL BUSINESS MACHINES CORPORATION. Aug.4, 1964 [July 31, 1963], No.30674/64. Heading G1U. [Also in Division H4] A multiple-subscriber, random access communication system in which each subscriber is assigned a unique pseudo-random signal waveform, comprises a plurality of transceivers, one for each subscriber, each transceiver including a waveform recognition device responsive only to its assigned pseudo-random waveform out of a group of such waveforms, and means at each transceiver for coding any message destined for one of the subscribers into pulse form and further coding each pulse into a particular sequence of binary pulses which is transmitted as the unique waveform assigned to that subscriber. The system uses matched filter and correlation techniques and enables signals to be transmitted which overlap in time and frequency even in the presence of noise. The pseudo-random waveforms are arranged to have a low crosscorrelation function and are termed quasiorthogonal. Transmitter. As shown in Fig. 3A, a voice signal at 50 position modulates pulses from a generator 30 to supply P. P. M. train 52 to a gating network 36 controlled by a flip-flop 38 to pass either the P. P. M. message signal 52 or a signal from acquisition calling network 40 to a pseudo-noise generator 42. P. N. G. 42 converts the calling signal or the message signal into a first or a second waveform assigned to the called subscriber depending upon whether an acquisition loop 49 or message loop 47 is activated. To call a subscriber the called party's address in terms of a 17 bit binary code, for example, is entered via lead 45 into a register 44 which controls P. N. G. 42, and the acquisition network receives a signal via line 41 to actuate shift register 40b to store the 17 bit code representing the calling party's address. The code signal stored by shift register 40b is supplied to gating network 36 which is conditioned initially by flip-flop 38 to pass this code signal to P. N. G. 42. The arrangement is such that loop 49 is first activated and the called party's address signal waveform is transmitted, and then the loop 47 is activated and the calling party's address signal using the message signal waveform assigned to the called subscriber is transmitted. After the calling party's address has been transmitted and a confirmation signal has been received over lead 35 from the called subscriber, a signal from shift register 40b operates flip-flop 38 so that the P. P. M. signals 52 are converted into the message signal waveform which is shown as the pseudo-random signal sequence 42a which is generated in response to every message pulse. Each pseudo-random sequence is spaced from the next sequence in accordance with the spacing of the corresponding pulses of the P. P. M. signal 52 and is supplied via a balanced modulator 71, mixer 73 and amplifier 79 to the transmitting aerial. The pseudo-random waveforms may be provided by a shift register provided with feedback from a selected number of the stages, Fig. 4A (not shown), Stepping of the shift register being gated by the P. P. M. signals. The mixer 73 is controlled by a channel hopping unit 77 so that after the calling party's address has been transmitted over a special channel allotted for this purpose, a pseudo-noise generator 142 supplies a sequence of signals to a shift register 146 which via a digital-to-analogue convertor varies the frequency of oscillator 150. In this way all message signals are transmitted over randomly varying channels within the wideband channel. A complementary control signal is supplied via a digital-to-analogue converter 149 to the associated receiver at the calling station. When a calling party starts a calling procedure its own address is loaded into a shift register 144 via line 145 and this controls the starting point of the pseudo-random sequence from P. M. G. 142 when a start signal appears on lead 134. Receiver, Figs. 3B, C. An acquisition receiver 100 is responsive to the calling and other supervisory signals which after limiting at 112a are supplied via a narrow band I. F. amplifier to a digital matched filter 116 which is responsive to both the acquisition code and the message code of the subscriber. The filter 116 Figs. 6A to E (not shown) is of the type comprising a shift register (81) in which the output of each stage is fed to network (83) providing the sum of all the voltages appearing across weighting resistors (82a to 82n) and producing an output on line (89) via a threshold circuit (87). A sampler and clipper (84) receives the input signal (Fig. 6B) and applies it to the shift register under the control of timing pulses (Fig. 6C). The input to the shift register is in the form of a series of positive and negative spikes (Fig. 6D) and when a complete waveform matched to the filter has been received a sharp peak (90) (Fig. 6E) is developed and passed by threshold circuit (87). The output of the filter 116, Fig. 3B in response to the acquisition signal is supplied via threshold device 120 to a gating circuit 124 controlling at 126 the sampling of the message signals which appear on output 117 of the matched filter 116 to apply these signals which constitute the calling party's address, via a further threshold circuit 128 and lead 130 to address register 44, Fig. 3A. The output of circuit 124 is also supplied via lead 134 to P. N. G. 142 of the channel hopping unit 77 so that the calling party's address stored at 44 is transmitted back as a confirmation signal in the message band. The message signal is received by a receiver 102, Fig. 3C including a mixer 113 which is controlled by signals from the associated channel hopping unit 77, a narrow band I. F. amplifier 115, a digital matched filter 119 and a demodulator 121 for the P. P. M. signals. At the calling party's message receiver 102 the confirmation signal generates a signal on line 133 for application via lead 35 to the gating circuit 36 and also via lead 134 to start P. N. G. 142. The calling party's address has also been loaded via line 130 into shift register 144 of the called party to control the starting point of its P.N.G. 142, in this case the mixer 73 being controlled by digital-to-analogue converter 149 and mixer 113 by digital-to-analogue converter 148. The demodulator 121 may be a synchronous detector, Fig. 3D (not shown).